MATERIAL PROPERTIES OF A FABRIC SHEET MOLDING COMPOUND FOR A STRUCTURAL COMPOSITE UNDERBODY. Libby Berger General Motors Research and Development

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1 MATERIAL PROPERTIES OF A FABRIC SHEET MOLDING COMPOUND FOR A STRUCTURAL COMPOSITE UNDERBODY Libby Berger General Motors Research and Development Charles Knakal Meda Engineering and Technical Services LLC Chris Morin ASG Renaissance LLC The Automotive Composites Consortium (ACC) has selected a fabric sheet molding compound (SMC) as the main material and process system for a structural composite underbody. This paper describes the properties of this SMC material, including tensile, compression, and flex. Thermal properties including coefficient of linear thermal expansion and Tg temperatures were determined. The effect of two different fabric weights, and several layups, thicknesses, and molding parameters is also reported. Overlap and butt joints within the layup were compared. The Automotive Composites Consortium is a joint program between General Motors, Ford, Chrysler, in partnership with the United States Department of Energy. Introduction One facet of the ACC Focal Project 4 is a the design, fabrication, and testing of a structural composite underbody. This project has been reviewed several times at Society of Plastics Engineers Automotive Composites Conference and Expositions and other conferences [1-11], and in several companion papers at this conference [12-15]. The development of a glass fabric SMC material and process system was crucial to this project. In this paper, we describe the material properties of this system, for our standard layup and for several variants. This material was initially developed at Polywheels in Oakville, ON. They used an 1854 glass fabric from Fiber Glass Industries in Amsterdam, NY, which is a basket weave, nominally 58 g/m2, with 5 strands in the warp direction and 3 ½ in the weft. This was compounded with a standard SMC machine, using a structural vinyl ester resin. Plaques were molded at 15 C in a 6mm x 6mm plaque tool, with vacuum. Normal molding time was 3 minutes, although this was varied as will be described later in this paper. When Polywheels ceased production, the project moved to Continental Structural Plastics (CSP) in Troy, MI, and Van Wert, OH, with the same glass fabric and a similar vinyl ester resin. Our standard layup is referred to as QQI, for quasi-quasi-isotropic, and is a 7-layer, slightly unbalanced system of 45//-45/9/-45//45. This layup was selected early in the project, because 7 layers gave us about the right thickness (3 mm) to compare with historical ACC data on other composite materials. The bias toward 45 was because our early modeling of the underbody showed a bias towards torsional loading. As we went forward with the project, we also used a number of different layups that would be representative of different areas in our underbody design. The layup design of the underbody is shown in Figure 1. The layup varies from four to twelve layers, with thickness designs of 1.8 mm to 5.4 mm. In addition, several areas had a low-density SMC (LD SMC) core. We made plaques of the more extreme of these layups, with some of the material properties reported herein. Other issues that we endeavored to address with plaques were the Page 1

2 effect of using a heavier fabric, how butt joints and overlap joints affected the properties of the composite, the single- properties to be used in finite element modeling, and the effect of cure time. Figure 1: Layup design for the Structural Composite Underbody. Properties Physical, thermal, and mechanical properties were measured in accordance with the ACC standard material guide [16]. The properties reported include density, fiber loading, tensile strength, modulus, and strain, compressive strength and modulus, coefficient of thermal expansion (CLTE) and thermal transitions as measured by dynamic mechanical analysis (DMA). Page 2

3 Material Properties and Characterization Standard QQI Material Physical and Mechanical Properties: Mechanical and physical properties for the QQI are given as an average over five separate batches of material, including compounding and molding trials. These five batches have an average glass fiber loading of 62.1 wt%, with a coefficient of variation (COV) of 5%, and an average specific gravity of 1.88, with a COV of 2%. The tensile and compressive properties are in Tables I and II. The mechanical properties are given as a function of the sample orientation within the plaque. While the plaque is a quasiquasi-isotropic layup, as given above, there are still differences in the directions. The fabric itself is unbalanced, with about 3% more fibers in the warp direction than the weft. There are also 4 layers of + 45 plies for 3 layers of /9 plies, also adding to the mild anisotropy. For these five batches, the overall tensile strength was 238 MPa and the tensile modulus was 18.9 GPa. The compression values were a little higher, with strength of 252 MPa and modulus of 21.1 MPa. Table I. Tensile properties for the 1854 glass fabric SMC QQI material, as an average of five batches. Tensile Strength (MPa) Tensile Modulus (GPa) % Strain Orientation Average COV Average COV Average COV % % % % % % % % % % % % Overall Fiber content wt% Specific gravity Table II. Compressive properties for the 1854 glass fabric SMC QQI material, as an average of five batches. Compressive Strength (MPa) Compressive Modulus (GPa) Orientation Average COV Average COV % % % % % % % 21. 4% overall Page 3

4 One of the batches of QQI material was characterized for tensile and compressive properties at -4 C and 8 C, as well as room temperature. As can be seen in Table III, this batch was one of the lowest fiber content of the five batches included in Tables I and II, which tends to increase the effect of temperature, since resin is more sensitive to temperature change in this range than is glass. The trends in this data are as expected, with both tensile and compressive strength and modulus slightly higher at -4 C, and lower at 8 C. Table III. Mechanical properties as a function of temperature for one batch of QQI material. Characterization by Delsen Laboratories. -4 C 23 C 8 C Tensile Strength (MPa) Tensile Modulus (GPa) Tensile Strain (%) Poisson s Ratio Compressive Strength (MPa) Compressive Modulus (GPa) Compressive Strain (%) Glass fiber content (%) 58 Specific Gravity 1.81 Thermal Properties: The thermal transitions for the QQI, as measured by DMA, are shown in Figure 2. We measure four transitions for this material: Tonset, the beginning of the steep decline in the storage modulus curve, T75, the temperature at which the storage modulus has dropped to 75% of its value at 25 C, and Tloss 1 and Tloss 2, two peaks on the loss modulus curve. The lower of the Tloss peaks is likely due to a secondary resin component, such as a low profile additive, with Tloss 2 describing the bulk of the material. For several plaques from one molding trial, the average values are shown in Table IV. Figure 3 shows the effect of the sample orientation on the DMA transitions. While these thermal transitions are primarily dependent on the resin, the amount and orientation of the reinforcement does have a small effect. This is best seen in the two transitions from the storage modulus curve, Tonset and T75, where the greater amount of 45 directional fibers, compared to and 9 fibers, slightly mitigates the effect of the softening resin. Page 4

5 Figure 2. DMA curve for the QQI material at standard molding conditions, showing the four thermal transitions. Table IV. Thermal transitions for QQI. Transition Temperature C T onset 113 T T loss T loss The CLTE is determined as the slope of a length vs. temperature curve, and is usually calculated in three temperature regions, as given in Table V. Figure 4 shows these curves for the QQI material. The CLTE values are close to expected values. Page 5

6 Thermal Transition, C Effect of orientation on thermal transitions Sample orientation T(onset) T(75) T(loss 1) T(loss 2) Figure 3. The effect of sample orientation on thermal transitions in the glass fabric SMC QQI as measured by DMA. Table V. CLTE for QQI fabric SMC, as a function of sample orientation. The units are C -1. Orientation -3 to 3 C 3 to 8 C 8 to 13 C 13.5E E-6 1.3E E E E E E-6 1.5E E E E-6 overall 14.6E E E-6 Fabric strand orientation One of the manufacturing issues was keeping the fiber strand direction straight. The weft strands tended to become further off-axis as the glass fabric was wound onto the roll, probably as a result of the tension being a little off. This translated to the fabric SMC being slightly off axis, and our final composite having inconsistent angles. In one set of experiments, we hand straightened each of fabric SMC to as close to warp and 9 weft as we could get. Tables VI and VII compare a ()4 layup, molded without straightening, to the same material with the plies straightened by hand pulling. The average values of the as-is material compared to the hand straightened material are not significantly different in most cases, given the high COV s of the as-is material. However, the as-is material had some extremely large COV s, while the straightened material has markedly lower variation. Page 6

7 Change in length, mm Temperature C Figure 4. The effect of test sample orientation of QQI fabric SMC on CLTE. The CLTE is the slope of the length expansion curve. Weight of the Fabric In order to see the affect of the fabric weight, we made glass fabric SMC with an FGI 2454 Rovcloth, to compare with the The 2454 is an unbalanced plain weave, nominally 5 strands warp and 4 weft, with an areal density of 79 g/m2, compared to nominal 5 strands warp and 3½ weft with areal density of 58 g/m2 for the In contrast to the 1854 fabric, the 2454 fabric showed little strand orientation disparity. Because the strands are heavier, the fabric is thicker (1.1 mm compared to.76 mm nominal for the 1854), and thus the strand z- direction deformation in the weave is larger. This was thought to potentially lead to lower tensile and compressive values, particularly for compressive strength. However, the values are not significantly different from the straightened 1854 numbers, although in most cases the 2454 strengths are slightly lower and moduli slightly higher (Figure 5). Page 7

8 Table VI. Material property results for () 4 layup using 1854 glass fabric which has not been straightened, and thus is randomly off-axis. (as is) Tensile Strength (MPa) Tensile Modulus (GPa) % Strain Orientation Average COV Average COV Average COV % % % % % 1.9 2% % % % % % overall (as is) Compressive Strength (MPa) Orientation Average COV % % % % overall 199 Table VII. Material property results for () 4 layup using 1854 glass fabric which has been straightened by hand pulling of the plies before molding. (straightened) Tensile Strength (MPa) Tensile Modulus (GPa) % Strain Orientation Average COV Average COV Average COV 246 5% % % % % % % % % % % % overall (straightened) Compressive Strength (MPa) Orientation Average COV 264 5% % % % overall 199 Page 8

9 Modulus in GPa Strength in Mpa 3 25 Strength of 1854 and 2454 Composites x4 tens QQI ten x4 com QQI com 25 2 Modulus of 1854 and 2454 Composites x4 tens QQI ten x4 com QQI com Figure 5. Strength and modulus comparison of fabric SMC made with 1854 or 2454 glass fabric. The 2454 fabric also has the advantage of higher resin flow in compounding, since it is a thicker fabric. This puts it closer to the design targets of the compounder, enabling better control of the resin content. Difference in Cure Time In order to better understand the processing rate of this material, we varied the cure time from 9 to 3 seconds, using the 1854 QQI material. Samples were molded at 15 C, with vacuum. Tensile and compression measurements were made, as well as thermal transitions using the DMA. All samples were taken in the orientation. The results of both the mechanical properties and the thermal transitions are shown in Figures 6 and 7. This demonstrates that for this particular resin formulation, there is little physical effect of curing the material for longer than 9 seconds. Page 9

10 Thermal transition, C Strength (Mpa) Modulus (Gpa) 3 Effect of cure time on mechanical properties tensile strength compressive strength tensile modulus compressive modulus Cure time (sec) Figure 6. Effect of cure time at 15 C on the tensile and compressive properties of glass fabric SMC. Effect of cure time on thermal transitions Cure time, seconds at 15 C Onset T 75 T loss 1 T loss 2 Figure 7. Thermal transitions as determined by DMA for a range of cure times at 15 C.. Fabric Joints Since the underbody is a very large part, it is necessary for us to split some of the plies into two or more pieces to fit on the fabric width, particularly for the 45 plies. In order to understand the effect of the joints between pieces, a brief study was made on 4 layer plaques (45///45) made with 2454 glass mat fabric. These had either a butt joint or an overlap joint on one of the plies, either the top layer (a 45 ) or the second layer (a ). Page 1

11 Figure 8 shows the tensile strength and modulus of samples taken across the joints in these plaques, and compared to an unjointed section of the same plaque. While tensile modulus does not suffer significantly from these joints, the tensile strength does show a significant decrease. Overlap joints are less detrimental than butt joints. This will have to be further studied to determine the extent of this, as well as how joints can be better positioned in the design of a part. Tensile strength of jointed materials Tensile modulus of jointed materials Unjointed Unjointed Jointed Jointed Overlap, surface Overlap, center Butt, surface Butt, center Overlap, surface Overlap, center Butt, surface Butt, center Figure 8. Effect of butt and overlap joints on the tensile properties of fabric SMC plaques. Summary This paper has presented the results of the materials development of a glass fabric SMC for the ACC Structural Composite Underbody project. This material and process system has been shown to be versatile in terms of material and layup, and as well as having good mechanical and thermal properties. Further development may be needed on reproducibility, especially in terms of resin content, as well as joint design. Acknowledgements The entire ACC Composite Underbody team is gratefully acknowledged. Particular thanks go to Hannes Fuchs (Multimatic Engineering Services), Bhavesh Shah and Dan Simon (General Motors). Continental Structural Plastics has been essential for the compounding of the material, and Century Tool and Gage is gratefully acknowledged for molding. This material is based upon work supported by the Department of Energy National Energy Technology Laboratory under Award Numbers DE-FC26-2OR2291 and DE-EE3583. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or im its endorsement, Page 11

12 recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References 1. L. Berger, et al., Materials and Processes for a Structural Composite Underbody, SAMPE Fall Technical Conference, Memphis, TN, 9/1/28 2. H. Fuchs, Initial Design of the Automotive Composites Consortium Structural Composite Underbody, SAMPE Fall Technical Conference, Memphis, TN, 9/1/28 3. L. Berger, et al., Development of a Structural Composite Underbody, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, 9/16/28 4. L. Berger, H. Fuchs, Automotive Composites Consortium Structural Composite Underbody, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, 9/16/ Berger, L., et al., Properties and Molding of a Fabric SMC for a Structural Composite Underbody, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy MI, Sept , Dove, C., Shear Deformation Properties of Glass-Fabric Sheet Molding Compound, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy MI, Sept , Fuchs, H., P. Deslauriers, Double Dome Structural Test-Analysis Correlation Studies, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy MI, Sept , J. Sherwood, Mesoscopic Finite Element Simulation of the Compression Forming of Sheet Molding Compound Woven-Fabric Composites, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, 9/16/ Jauffres D., et al., Mesosocopic finite element modeling of woven reinforcements applied to sheet molding compound forming simulation, Proceedings of the 17th International Conference on Composite Materials. Edinburgh, UK, Shah, B., et al., Structural Performance Evaluation of Composite-to-Steel Weld Bonded Joint, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy MI, Sept , Fuchs, H., B. Conrod, Super Lap Shear Joint Structural Test-Analysis Correlation Studies, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy MI, Sept , L. Berger, Program Summary of the ACC Automotive Composites Underbody, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, Sept , D. Q. Houston, et al., Fatigue Performance of SMC Composite Material under Different Environmental Damage and Temperature Conditions, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, Sept , 211. Page 12

13 14. C. Knakal, et al., Manufacturing Scenarios and Challenges with a Fabric SMC Automotive Underbody, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, Sept , H. Fuchs, Gillund, E., Composite Underbody Component and Assembly Structural Test- Analysis Correlation, Society of Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, Sept , SAE J2253, December, Page 13